In the manufacture of ordinary paper and paperboard, so-called sizing is effected in order to provide the products with resistance to blotting, water resistance, waterproofness, etc. Such sizing includes so-called internal sizing in which a sizing agent is added to a pulp furnish during the papermaking process before a fiber sheet is formed and so-called surface sizing in which a sizing agent is applied on the surface of formed fiber sheet.
In internal sizing, conventionally there is employed an acid sizing method in which sizing is carried out in a pH range of 4.5-6.5 using a rosin sizing agent and crude aluminum sulfate. Recently, so-called neutral sizing is attracting attention. This sizing is carried out in a neutral or slightly alkaline pH range of 6.5-9, with the aim of using inexpensive calcium carbonate as a filler, using brokes or waste paper containing calcium carbonate, employing closed system in which water for papermaking is recycled, and producing permanent paper for books and documents.
Today, ketene dimer compounds, substituted cyclic dicarboxylic acid anhydride compounds, copolymers of a cationic monomer and a hydrophobic monomer, cationized petroleum resins, cationized aliphatic amides, etc. are used as sizing agents. Among these, aqueous dispersions of ketene dimer compounds are most widely used because of their superior sizing effect.
Conventionally, ketene dimer compounds are marketed and used in the form of aqueous dispersions in which the ketene dimers are dispersed together with starch, especially cationized starch, in a continuous aqueous phase. However, ketene dimer compounds are inherently reactive with water and are difficult to be provided as stable aqueous dispersions. Often these dispersions lose homogeneity and gel or form deposits during storage and thus lose their paper-sizing effect and waterproofing effect. It is very difficult to provide an aqueous dispersion sizing agent which is of high concentration, stable at high temperatures and mechanically stable.
In Japanese Laid-Open Patent Publication No. 60-258244(1985), a method for dispersing ketene dimer compounds in an aqueous continuous phase containing an acrylamide polymer having cationic groups is disclosed. However, the internal sizing composition in accordance with this method is still unsatisfactory in mechanical stability, storage stability, sizing effect, etc.
Meanwhile, surface sizing is advantageous in that it is not influenced by the quality and temperature of the water used in the papermaking and by the acidity or basicity (pH) of the water used in papermaking and enables economical use of the sizing agent and suitable process control.
As surface sizing agents, anionic water-soluble high polymers such as oxidized starch, phosphoric acid derivatives of starch, carboxymethyl cellulose, poly(vinyl alcohol), anionic acrylamide polymers, anionic styrene polymers, etc. are used. These anionic water-soluble high polymers react with aluminum ions of aluminum sulfate and become hydrophobic, whereby they exhibit sizing effect when applied to acidic paper in which aluminum sulfate is used. However, these anionic water-soluble high polymer sizing agents cannot exhibit sizing effect for neutral paper in which little or no aluminum sulfate is used.
Aqueous dispersions of ketene dimer compounds, which are commercially available today as neutral surface sizing agents, are known as sizing agents which exhibit excellent sizing effect not only for neutral paper but also for acidic paper.
In surface sizing, the sizing agent is prepared into a surface sizing solution under warmed condition, and the thus prepared surface sizing solution is recycled in use, during which the liquid suffers from the effects of heat and mechanical impact. Therefore, the surface sizing agent must be provided with resistance to heat and mechanical impact and low foaming property when it is recycled in use.
In surface sizing, the sizing agent is, on some occasions, required to provide paper with surface bonding strength and good printability in addition to blotting resistance. The above-mentioned anionic high polymers are used for such purpose. Therefore, the surface sizing agent is required to be provided with compatibility with such anionic water-soluble high polymers.
The ketene dimer composition in accordance with the above-mentioned Japanese Laid-Open Patent Publication No. 60-258244 is still unsatisfactory in compatibility with the above-mentioned anionic water-soluble high polymers.